Update PR checklist + CONTRIBUTING.md (#1220)

Author: marc-vdm

.github/PULL_REQUEST_TEMPLATE.md

@@ -18,6 +18,7 @@ pull-request is published.
       `bug`, `feature`, `ui`, `change`, `documentation`, `breaking`, `ci`
       as they show up in the changelog.
 - [ ] Link this PR to related issues by using [closing keywords](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue).
+- [ ] Add a milestone to the PR for the intended release.
 - [ ] Request a review from another developer.
 
 <!--

CONTRIBUTING.md

@@ -168,7 +168,7 @@ Fork the AB repository, create a new branch from the `master` branch, apply your
 forked repository and open a pull request from there. 
 When creating a pull-request, please add a description of what your new changes should accomplish (and if this wasn't a 
 known problem, _why_ AB needs this).
-If it's a larger pull-request, feel free to add screenshots or other useful descriptions to help us understand what 
+If it's a larger pull-request, feel free to add screenshots/gifs or other useful descriptions to help us understand what 
 you did. 
 Next, make sure your pull-request has a sensible title that _users_ can easily understand, this name is shown in the 
 changelog of the next release.
@@ -185,7 +185,9 @@ This is a [GOOD example of a title](https://github.com/LCA-ActivityBrowser/activ
 ![image](https://github.com/LCA-ActivityBrowser/activity-browser/assets/34626062/0822a3c8-d01c-4a0c-8cbc-d44c4bbddc02)
 - This is a good example because it describes what is changed.
 
-Finally, please make sure you follow the pull-request checklist (you can see this when you open a pull-request.
+Finally, please make sure you follow the pull-request checklist (you can see this when you open a pull-request).
+If you can't complete certain tasks because you don't have write-access (e.g. adding a label and milestone), 
+they will be performed by the maintainers.
 
 Upon creating a new pull-request, there are a number of automated tests that will be run. 
 If the test-runners fail in some way, it is likely that there is an error somewhere in your PR.